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Fig. 8.—Portion of an older root of an oak, which had penetrated while young between two pieces of hard rock, and had to adapt its form accordingly as it thickened. (After Döbner.)
It has already been mentioned that the tip of the young primary root circumnutates, and Darwin also found that the tip of the radicle is extremely sensitive to the irritation of small bodies in contact with it. It is also positively geotropic, directing itself vertically downward if the partially grown radicle is laid horizontally; and it may be assumed from the behavior of other plants of the same kind that the tip of the radicle is negatively heliotropic—i.e., it turns away from the source of light. Whether it is also sensitive to differences in the degree of moisture on different sides (hydrotropic), or to differences of temperature (thermotropic), is not known, but it may be inferred that such is the case; nor do we know whether it is affected by electric currents in the earth